Tip/tilt/piston (TTP) micro-mirror arrays are known for their ability to simultaneously direct multiple beams of light in various directions at high speeds (kHz), and have been used for various applications, including for example optical routing/cross-connects, image projection, object tracking, adaptive optics for aberration correction and laser scanning/steering. And existing micro-mirror designs exhibit a wide range of mirror scales and discrete/continuous configurations, actuation physics/designs, and bearing designs, with the actuation physics typically setting bounds on the speed of mirror actuation, while the discrete or continuous surface design of the mirror configuration often placing range restrictions on the mirror motions.
One example type is a Lorentz-actuated micro-mirror array using magnets and coils [10] provided as conductive traces on each mirror for directly controlling mirror motion. The Lorentz energy densities, however, are typically low (e.g. in the range of ≈10-100 pJ/mm3), resulting in slower (e.g. <1 kHz) stepping rates. In addition, such Lorentz actuated mirror designs often use a gimbaled mirror, which allows for tip/tilt motion, often to large angles (150) but with the drawback of poor dynamics and with reduced fill factor (i.e. low directed light) below acceptable levels due to the intermediary structure.
Electrostatic actuators are also used in micro-mirror arrays, as it produces relatively high forces without significant dynamics or heating problems. Capacitive comb drives are known to exhibit some of the highest force densities measured due to the ability to pack combs very efficiently. This produces very fast mirrors, but with the necessity of decoupled actuation for multi-axis designs, as the combs must only move in the prescribed, often 1 DOF, motion to avoid snap in. This results in more complex designs, often utilizing transmissions. Capacitive parallel plate drives offer a tradeoff of lower forces (and thus slower mirrors) but with significant reduction in device complexity due to the simpler actuator geometry and the ability to do direct multi-axis actuation without transmissions.
Continuous plate devices are designed specifically for wavefront phase modulation, and typically use parallel plate or pizeo arrayed actuation. The continuous reflective surface allows for spatially smooth adjustment of beam phase in-between actuation points, however it limits net tip/tilt of the mirror to generally well below 1, strongly limiting their utility beyond aberration correction.
Thermal devices have been shown to produce extremely large range motion (90°) due to the generation of thermal stress in the mirror flexural bearing. This is done through Ohmic heating of the bearings. This method generates two problems: structural warping due to heating and lower response rates, generally <2 kHz.
Piezo surface film devices (7) derive actuation effort through electrically induced surface stress on beams. This avoids the structural change due to thermal loading, allowing for greater stability. The nature of the piezo films as thin depositions limits the max voltage and thus the max force generation. This typically relegates piezo actuation to resonant or low speed devices. Piezos also suffer from a slow creep when operating in open loop conditions, making it difficult to generate precision motions.
What is needed therefore is a tip-tilt-piston micro-array capable of attaining high speeds over large ranges in all axes (i.e., tip/tilt/piston) to be accurately controllable for fine resolution mirror positioning, especially for use in demanding environments and applications, such as in light shield systems envisioned for bullet defeat applications, and in nano- and micro-fabrication systems.